Known cannulae generally comprise a two-piece body, to the front part of which belong wings for fastening the cannula on the patient's skin for instance with adhesive tape, a point of support for the operator's forefinger for handling the cannula (a capped injection valve for medicine injections often serves as such point of support), a flexible catheter through which the liquid is infused or injected into the vein, and fittings (a so-called Luer lock fitting) for the rear plug or for the supply hose of the liquid to be infused.
The rear part of the body consists of a so-called indication chamber that can be uncoupled from the former, which shows the flowing of blood into the cannula and through which the cannula can be deaerated before treatment. At the front of the chamber is also fastened a hollow steel needle, or a steel cannula, with which the vein will be punctured and which part of the body the operator pulls out from the vein and from the cannula hub, leaving the flexible catheter in the vein. The rear part is also provided with a vertical support plate mainly for the thumb.
Cannulae of this kind have several disadvantages. First, the operator's hold mainly between the thumb and the forefinger is not the best possible for giving injections because it easily happens that the steel needle's direction of movement deviates from both the operator's aiming line and his hand's direction of movement, and so practice is needed to find the right puncture point and angle.
Second, the injection valve serving as the forefinger's point of support cannot be shaped more friendly for the operator's hold because usually such valves are of standard make. The smallest cannulae have no room at all for such valves, so it has been necessary to provide them with separate support plates, which increases the cost of such cannulae and eliminates in them the possibility of giving medicaments in the form of injections.
As a third disadvantage could be mentioned that the rear plug of the cannula is usually fastened either on a wing on the cannula's front part or behind the cannula's rear part, that is the indication chamber, in which cases it cannot be removed with the hand that holds the cannula. As the other hand is busy enough, for instance pressing the punctured area while the steel needle is being pulled out, calming down the patient, and so on, to remove the plug from its holder and insert it into the opening is a difficult operation.